1. Field of the Invention
This invention relates generally to liquid crystal display (LCD) devices and light emitting structures (LESs), such as light emitting diodes (LEDs) and light emitting polymer (LEP) devices.
2. Related Art
Liquid Crystal Displays are used in many practical display systems including display systems for laptop computers, calculators, personal digital assistants, and cellular telephones. An LCD comprises at least one liquid crystal (LC) cell. The LC cell typically has a layered construction including upper and lower spaced-apart transparent substrates, often made of glass, having inner surfaces encapsulating an LC layer. A transparent conductive layer or electrode, typically made of Indium Tin Oxide (ITO), and a transparent polymer both coat the respective inner surfaces of the transparent substrate layers. The polymer coatings or aligning layers align the orientations of LC molecules in the LC layer and adjacent the aligning layers. A voltage applied across the conductive layers or electrodes establishes an electric field across the LC layer and thereby further controls the LC molecule orientations throughout the LC layer, as is known, to control light passing through the LC cell.
Known LCD device configurations include passive and active LCD matrix displays. Such displays typically include a matrix of transparent ITO pixel electrodes or electrode regions arranged in addressable rows and columns. Each pixel electrode or electrode region defines an image pixel that can be selectively driven, that is, turned on or off by a voltage signal applied to the pixel electrode or electrode region.
As pixel densities increase, the sizes of the pixel electrodes and electrode regions correspondingly decrease. As a result, higher voltages are required to rapidly drive the pixel electrodes or electrode regions. Higher voltages disadvantageously consume more power. Also, as the number of pixels increases with display size and increased pixel density, even more power is required to drive a higher number of pixel electrodes covering a larger area of the LCD.
In matrix LCD applications, scanning voltage signals used to select and drive the pixel electrodes operate at relatively high frequencies, for example, in color television applications, pixel electrodes are scanned at frequencies of up to 23 MHZ. Also, there is a constant push to increase refresh rates in many types of LCD devices. There is therefor an ever increasing need for faster and more responsive LCD devices operating with reduced power consumption.
One factor determining LCD responsiveness and power consumption performance is the electron mobility and thus conductivity of the transparent pixel electrodes used in the LCD device. For example, an increase in pixel electrode conductivity results in a decrease in the voltage signal level required to drive the pixel electrodes and a corresponding decrease in LCD display power consumption. Such an increase in conductivity also results in a corresponding increase in the responsiveness of the pixel electrode to voltage signals applied thereto. The use of semi-transparent or transparent semiconductors/metal-oxides, such as ITO, as the preferred materials for the pixel electrodes in known LCD devices disadvantageously limits the responsiveness of such pixel electrodes, and also disadvantageously limits the ability of the LCD devices using such pixel electrodes to reduce power consumption. Such disadvantages result from the relatively poor conductivity of the semiconductor material in relation to other materials, such as metals. For example, the conductivity of a semiconductor material such as ITO is between 100 and 1000 times less than the conductivity of a metal.
Another known problem associated with LCD displays is an annoying display glare caused in part by the undesirable reflection of visible light from the surfaces of the transparent semiconductor/metal-oxide electrodes.
Accordingly, there is a need to increase responsiveness and reduce power consumption in LCD display devices.
There is a further need to reduce undesired display glare in the LCD devices.
Light emitting structures, such as LEDs and LEPs, are also used in many practical display systems including display systems for laptop computers, calculators, personal digital assistants, and cellular telephones. A light emitting structure (LES) typically comprises spaced-apart cathode and semi-transparent anode electrodes and an active layer between the spaced electrodes. In the case of an LED, the active layer includes an active semiconductor layer. In the case of a light emitting polymer (LEP) device, the active layer includes an active polymer. When voltages are applied to the anode and cathode electrodes, a voltage potential difference develops across the active semiconductor or polymer layer and causes the active layer to emit visible light. The emitted visible light is at least partially transmitted through the semi-transparent anode. A known LED for emitting light, such as blue light, includes an anode electrode made of a semi-transparent semiconductor, such as ITO. Use of the ITO composition has disadvantages as mentioned above in connection with LCDs.
A typical LEP structure is similar in nature to an LED. That is, it is composed of a polymer material sandwiched between two electrodes. In a known LEP, one of the two electrodes can be ITO, while the other can be a silver mirror. LEPs have entered the scene more recently than LEDs and are sometimes preferred over LEDs because LEPs can be easier to manufacture than LEDs. However, the principles of operation for LEDs and LEP remains essentially the same, and their geometries are very similar. Use of the ITO composition in an LEP has disadvantages as mentioned above in connection with an LED and an LCD.
Therefore, there is a need to improve LES (for example, LED and LEP) device performance with respect to at least the semi-transparent anode electrode.